This invention relates to a compound oil ring and, more particularly, to improvements in a compound steel oil ring assembly having a spacer expander and side rails disposed above and below the spacer expander and brought into pressured contact with the inner wall of the cylinder of an internal combustion engine by the spacer expander.
In order to improve the wear resistance of the side rails in a compound oil ring assembly disposed in the oil ring groove of a piston, the conventional practice is to produce so called "chrome-plated side rails" each of which has its outer circumferential surface or both its outer and inner circumferential surfaces plated with chrome, and so called "nitrided side rails" each having its entire peripheral surface subjected to a nitriding treatment. These rails are installed in the internal combustion engine.
The amount of wear at the oil ring groove side surfaces produced by the contact between the nitrided side rails and chrome-plated side rails and these side surfaces is five times greater in the case of the nitrided side rails. In the case where the nitrided side rails are used, the deterioration in the side rail performance of the side rails due to wear is great and a large amount of oil is consumed.
It has been clarified that this wear phenomenon at the oil ring groove of the piston is characteristic of the nitrided side rails, and that groove wear is caused by the corners at the opening between the opposing ends of the nitrided side rails.
The nitrided side rails have a high surface hardness of Hv 1100-1200 and possess the aforementioned opening between the opposing ends of the rail. During operation of the engine, therefore, the opening vibrates so that its corners cut into the side surfaces of the piston oil ring groove, thereby causing wear.